As explained in the commonly assigned copending application Ser. No. 07/714,168, filed 12 Jun. 1991, particle board can be fabricated by spreading a mass of particles admixed with an adhesive or glue upon a conveyor belt to form a mat which can be compressed, e.g. in a double-belt press or a platen press, into particle board. The pressing can be carried out in the presence of steam which can serve to activate the adhesive or glue and/or with the aid of a thermally-activated adhesive or glue.
Reference may also be made to the spreading apparatus described in German patent document DE 25 35 461, published Feb. 10, 1977.
In that system, the mixture of the particles and the glue from a bunker or bin is discharged utilizing a discharge conveyor belt and a back-stripping arrangement which controls the deposit of the mass upon the conveyor belt to a discharge end of this conveyor which forms a material-discharge zone. The material passing off the end of the feed conveyor in this zone may pass in front of an air distributor at which the freely falling mass is laterally entrained by an air blast and distributed over a receiving surface in the form of the aforementioned mat-collecting conveyor belt.
In this air blast distributor, which forms an air-sifting system, the raw material is deflected by the air jets so that it falls in a number of parabolic paths which are distinguished by the fact that the path assumed by the material is dependent upon the mass of the material horizontally entrained by the air stream. In other words, heavier material tends to fall closer to the original line of free-fall at the discharge region while lighter materials are blown further away from the direct fall line.
The particular parabolic path assumed by materials of a particular weight is controlled by the velocity of the air. This method of spreading the particle mass is especially suitable for use when the material is to form a fine layer, e.g. as the upper or lower face layer of a mat and hence pressed board constituted of a plurality of layers, the inner layers of which can be somewhat coarser than the fine face layers.
Spreading systems of this type do not preclude the formation of glue clumps, aggregates or agglomerates in the raw material mass. Such glue clumps or balls are detrimental to the quality of the fabricated pressed board, especially when they appear in thin pressed board or the glue clumps or balls appear in the cover layers or face layers of the pressed board.
It is desirable, therefore, to remove these balls from the raw material so that the balls do not end up in the mats and especially do not appear in the face layers of multilayer mats.
The removal of such glue agglomerates or balls has been found to be especially important when the mats of material are to be pressed between steel belts of continuously operating presses because such balls have been found to be detrimental to the belts, especially by reducing the useful life thereof.
In the spreading system of German patent document DE 25 35 461, the raw material discharge region is fitted with a disk sieve constituted from a multiplicity of disk rollers which rotate in the same sense and whose disks interdigitate and at their peripheries can be toothed.
The chips pass between the rotating disks of the disk grate thus formed and depending upon the interdisk spacing, can be subjected to a classification into fine material, medium material and coarse material. If balls of glue or adhesive are present in the raw material, these can be separated from the chips or particles which trickle through the disk grate and can be entrained to a side of the disk grate whence they are discharged.
The air sifting of the downwardly trickling particulate material can be provided in this system below the disk grate, especially between the disk grate and the conveyor belt receiving the mat.
The air classification can result in entrainment of the finest particles to the upstream side of the system counter to the movement of this conveyor belt to enable the deposition of a fine cover or face layer on the latter. While this system operates effectively in most instances, when the combination of the disk grate with the air sifting is not used as is the case for the production of especially thin pressed board or when the spreading device is used only for the formation of the face layer, it is found that balls or agglomerates of glue are formed and deposited upon the conveyor belt which can be detrimental to the system.